Present-day continuous cyclic FCC processes utilize fluidizable catalyst particles containing a crystalline zeolitic aluminosilicate component (usually an ion-exchanged form of a synthetic crystalline faujasite) and a porous inorganic oxide matrix. This type of catalyst must be regenerated to low carbon levels, typically 0.5% or less, to assure that the catalyst particles possess desired activity and selectivity before the particles are recycled to a conversion zone for catalytic upgrading of hydrocarbon feedstock. In most regenerators the combustible solids deposited on the spent solid catalyst particles from the cracking zone are burned in a confined regeneration zone in the form of a fluidized bed which has a relatively high concentration of catalyst particles (dense phase). A region of lower solids concentration (dilute phase) is maintained above the dense phase. A typical regeneration cycle is described in U.S. Pat. No. 3,944,482 to Mitchell.
High residual concentration of carbon monoxide in flue gases from regenerators have been a problem since the inception of catalytic cracking processes. The evolution of FCC has resulted in the use of increasingly high temperatures in FCC regenerators in order to achieve the required low carbon levels in the regenerated crystalline aluminosilicate catalysts. Typically, present day regenerators now operate at temperatures in the range of about 1100.degree. F. to 1350.degree. F. when no promoter is used and result in flue gases having a CO.sub.2 /CO ratio in the range of 1.5 to 0.8. The oxidation of carbon monoxide is highly exothermic and can result in so-called "carbon monoxide afterburning" which can take place in the dilute catalyst phase, in the cyclones or in the flue gas lines. Afterburning has caused significant damage to plant equipment. On the other hand, unburned carbon monoxide in atmosphere-vented flue gases represents a loss of flue value and is ecologically undesirable.
Restrictions on the amount of carbon monoxide which can be exhausted into the atmosphere and the process advantages resulting from more complete oxidation of carbon monoxide have stimulated several approaches to the provision of means for achieving complete combustion of carbon monoxide in the regenerator.
The use of precious metals to catalyze oxidation of carbon monoxide in the regenerators of FCC units has gained broad commercial acceptance. Some of the history of this development is set forth in U.S. Pat. No. 4,171,286 and U.S. Pat. No. 4,222,856. In the earlier stages of the development, the precious metal was deposited on the particles of cracking catalyst. Present practice is generally to supply a promoter in the form of solid fluidizable particles containing a precious metal, such particles being physically separate from the particles of cracking catalyst. The precious metal, or compound thereof, is supported on particles of suitable carrier material and the promoter particles are usually introduced into the regenerator separately from the particles of cracking catalyst. The particles of promoter are not removed from the system as fines and are cocirculated with cracking catalyst particles during the cracking/stripping/regeneration cycles. Judgement of the CO combustion efficiency of a promoter is done by measuring the difference in temperature, delta T, between the (hotter) dilute phase and the dense phase.
There is a constant search for FCC CO promoters of higher activity and longer durability of unit retention. To be effective, a CO promoter must have physical properties that allow it to be retained within the FCC unit. High density CO promoter particles are desirable since they are less likely to be entrained in the flue gas and carried out through the cyclones. They are also more likely to promote CO combustion within the regenerator dense phase, where it is desired, than in the dilute phase, where it contributes to afterburning. The particle size distribution of the CO promoter microspheres is another important physical property since larger particles are more likely to be retained by cyclones than smaller ones. Similarly, very attrition resistant particles are desirable since they are less likely to generate fines which can be lost through the cyclones, carrying a portion of the CO promoter's activity with them.
CO promoters should, therefore, be as dense, coarse, and attrition resistant as the manufacturer can make them. These characteristics do not adversely affect catalyst fluidization or circulation, however, since the promoter concentration is so low (generally less than 1% of the catalyst inventory).
Promoter products formerly used on a commercial basis in FCC units include calcined spray dried porous microspheres of kaolin clay impregnated with a small amount (e.g., 100 or 500 ppm) of platinum. Reference is made to U.S. Pat. No. 4,171,286 (supra). Most commercially used promoters are obtained by impregnating a source of platinum on microspheres of high purity porous alumina, typically gamma alumina. The selection of platinum as the precious metal in various commercial products appears to reflect a preference for this metal that is consistent with prior art disclosures that platinum is the most effective group VIII metal for carbon monoxide oxidation promotion in FCC regenerators. See, for example, FIG. 3 in U.S. Pat. No. 4,107,032 and the same figure in U.S. Pat. No. 4,350,614. The figure illustrates the effect of increasing the concentration of various species of precious metal promoters from 0.5 to 10 ppm on CO.sub.2 /CO ratio.
Commonly assigned U.S. Pat. No. 4,608,357 (Silverman et al.) teaches that palladium is unusually effective in promoting the oxidation of carbon monoxide to carbon dioxide under conditions such as those that prevail in the regenerators of FCC units when the palladium is supported on particles of a specific form of silica-alumina, namely leached mullite. The palladium may be the sole catalytically active metal component of the promoter or it may be mixed with other metals such as platinum.
U.S. Pat. Nos. of Schwartz (4,350,614, 4,072,600 and 4,088,568) mention rare earth addition to Pt based CO promoters. An example is 4% REO that shows some advantage. There is no teaching of any particular effect of a particular RE metal or any difference(s) between different RE metals and how they influence promoter stability and performance.
Csicsery, in U.S. Pat. No. 4,137,151 discloses that lanthanum at concentrations of 0.05-10% is effective in FCC CO combustion when dispersed on the catalyst or associated with a refractory matrix material such as alumina. However, Csicsery does not teach CO promoters based on a precious metal such as platinum.
Brown et al., U.S. Pat. No. 4,839,026, discloses FCC catalysts active in CO combustion, which are based on precious metal supported on oxides, including alumina and cocirculated with separate particles of alumina treated with rare earths, including 48% mixtures of La and Ce. The rare earth, preferably cerium, is used for SOx pickup.
Ernest in U.S. Pat. No. 4,585,752 discloses a high temperature stable catalyst made of a base metal such as Cr, Hf and Nb or their oxides in a matrix of composite particles having a platinum group metal on a ceria promoted refractory oxide powder, such as alumina. The catalyst was found to catalyze combustion of hydrocarbon fuel. It is disclosed that the ceria enhances the dispersion and stability of the platinum group metal in the catalyst composition. There is no mention in U.S. Pat. No. 4,585,752 of a Ce-Pt catalyst without the base metal and there is no disclosure suggesting that the catalyst is effective in FCC CO promotion.
U.S. Pat. Nos. 4,350,614, 4,072,600 and 4,088,568, Schwartz, disclose the possibility of adding REC1.sub.3 6H.sub.2 O with a platinum promoter for FCC. Example 10 discloses a catalyst containing 4.2% REO. Example 14 (without RE) and 15 (with RE) show the advantage of using RE on CO combustion.
U.S. Pat. No. 4,350,615 (Meguerian et al.) teaches a CO combustion promoter composed of Pd-Ru which also enhances SOx removal.
Peters U.S. Pat. Nos. 5,164,072 and 5,110,780, relate to an FCC CO promoter having Pt on La-stabilized alumina, preferably about 4-8 weight percent La.sub.2 O.sub.3. It is disclosed that ceria "must be excluded". At col. 3, it is disclosed that "In the presence of an adequate amount of La.sub.2 O.sub.3, say about 6-8 percent, 2 percent Ce is useless. It is actually harmful if the La.sub.2 O.sub.3 is less." In an illustrative example Peters demonstrated an adverse effect of 8% Ce on CO promotion of platinum supported on a gamma alumina and a positive effect of La. It is noted that the calcination temperature used in illustrative examples in these patents was 1350.degree. F. Peters also cites previous work such as RE stabilized alumina and the use thereof in washcoat for auto exhaust catalyst application.
One of the best promoters commercially available prior to this invention is understood to be based on platinum dispersed over lanthanum (about 8% lanthana) supported on transition alumina.
By way of summary, the prior art does not teach that ceria would be useful in FCC CO promotion of Pt/Al.sub.2 O.sub.3 catalysts. The teachings of Peters are to the contrary.